I'm hoping that it is obvious that if the tires have different load carrying capacities, but are actually carrying the same load, the operating temperatures would be different. In other words, there are several factors at work here, not just brand.

An old wives tale. Nitrogen and air react very very similarly when used in tires.

To clarify....we used Nitrogen in Sprint Kart (go cart) racing to promote a more predictable handling setup in relatively short races, 15-30 minutes at speeds up to 100 mph. The tires swell, change shape at a lower rate with Nitrogen in those conditions.

Nitrogen expands at a slower rate than compressor air by nature primarily because pure nitrogen from a pressurized tank assumedly isn't contaminated by moisture (H20). Compressed air takes on whatever moisture exits in the environment and expands more rapidly when heated.

I'm not sure that you would see a significant benefit in trailers tires in the long run especially considering the hassle in maintaining a predictable Nitrogen supply.

I replaced a pair of 6 year old Marathons with Maxxis ST tires before my trip west.

1. I never had a problem with the Marathons, even though they are run at near full load capacity on a single axle Tradewind. Tire load is about 2450 lbs. Always ran 65 psi cold, although they read in the upper 70s when up to temp at 70 mph.

2. The new Maxxis have about 4000 miles on them. I keep them inflated to 75 psi. One of the wheels failed halfway across Montana and I replaced that wheel. The original wheels were filled with nitrogen, and the replacement wheel was filed with regular compressed air. I saw no difference in cold or hot pressure (up to 88 psi hot) between the nitrogen tire and air filled tire.

Your results may vary. I am a scientist and engineer so I understand that a single set of data points is not statistically valid.

The original wheels on my '59 Tradewind were 7.00-14.5 trailer wheels, which I replaced with Dexstar steel wheels rated 75 psi.

My tire retailer said they would be alright for the Maxxis 10 ply rated tires. Turns out he was wrong. One of the wheels split (slow leak at the weld), but I noticed it right away on my TPMS sensor. I was able to make it to a Tire-O-Rama with 67 psi left in the tire.

Now I have a white spoke wheel ($39.95) on one side and a black baby moon on the other.

One of the first things I did on our new 2012 was replace the factory tires with Michelin LTX M/S 2 P235/75R15XL. I had to remove the wheels/tires and take them into Costco to have the new Michelins mounted. I had nitrogen put in the tires and simply stop by Costco at the start of the season and have them checked. They have not lost more than a couple of pounds which is well within normal equipment error. I also had the cheap lug nuts replaced with solid chrome ones.

Why? I drive the speed limit which is more often than not over the 60mph rating. I have had two blowouts over the years with one causing a fair amount of damage that took a great deal of arguing with the insurance company to cover. I think it handles and stops better especially in the wet. It would also appear to ride softer.

I don't run monitors, measure air pressure/temps when I stop, or otherwise worry about them at all.